The invention is based on a priority application EP 06290148.3 which is hereby incorporated by reference.
The invention relates to a Radio-Frequency (RF) waveguide comprising at least a folded sheet.
Guiding high frequency or also called Radio Frequency (RF) electromagnetic waves takes place within transmission lines comprising e.g. a RF coaxial cable, an elliptical waveguide or another metallic tube or combinations hereof.
Today the necessary mechanical properties such as lateral pressure and tensile rigidity of RF-cables, particularly RF coaxial cables, and RF-waveguides, in the following exemplary embodiments described by the term “waveguide”, are achieved using electric conductors with diameters or wall thicknesses high enough to provide the required mechanical properties. The dimensions wall thickness and/or diameter of the electric conductors are significantly higher than required to fulfill the real function of transmitting high frequency signals. The dimensions required to fulfill the real function mentioned above are defined by the so-called skin deepness or by the so-called skin effect. Guiding particularly high frequency or RF signals in the form of electromagnetic waves within a waveguide takes place in a thin region close to the surface of the electric conductor. The orientation of the surface, e.g. regarding a RF coaxial cable the inner or the outer surface, beneath which guiding of electromagnetic waves takes place is defined by the arrangement of the electric conductors relative to each other.
Using solid electric conductors leads to high weight and high costs due to high portions of metal within the waveguide.
Rising prices for raw metals such as raw copper force manufacturers to reduce the portion of copper and other metallic components within waveguides to an absolute minimum and, at the same time, to keep at least the high-frequency specifications at today's values.
From DE 2 022 991 and from DE 20 56 352 it is known to form a waveguide made of a sheet of an electric conductor that is folded to a tubular or cylindrical conductor enclosing a core. A first the tubular conductor is formed by folding a metallic sheet having the form of a strip to a tube, wherein the inner diameter of the tubular conductor is slightly larger than the outer diameter of the core. The joint between the margin regions of the sheet that are adjacent after shaping the tubular conductor are welded to avert bunching when bending the waveguide. The core is made of a prefabricated solid or a hollow-cylindrical copolymer of ethylene. The tubular conductor after completing is pulled down on the core, wherein the electric conductor and the core are laminated with each other. Particularly to allow welding of the margin regions of the sheet, a higher material thickness is required than needed according to the electric boundary conditions. Furthermore, before laminating the tubular conductor and the core, the tubular conductor has to be formed to a plain ended pipe. This also requires a material thickness much higher than needed according to the electric boundary conditions. Furthermore, the manufacturing process to form a plain ended pipe is very costly and labor intensive.
From US 2003/0174030 A1 a RF coaxial cable with cladded, tubular conductors, as well as a RF-waveguide is known, wherein each conductor includes a base layer formed of a relatively higher conductivity metallic material, such as copper, silver, or gold and a bulk layer formed of a relatively lower conductivity metallic material such as aluminum or steel. Each one of the tubular conductors are made of a sheet in the form of a strip of bulk layer coated with the base layer. After coating, the sheet is folded to a tubular conductor enclosing a core, wherein the joint between the margin regions of the sheet that are adjacent after shaping the tubular conductor are welded to avert bunching when bending the coaxial cable. The coating takes place by cladding, electro-deposition, sputtering, plating or electro plating. The drawback of this solution is the relatively high weight of the tubular conductors, the usage of relatively expensive materials to form the tubular conductors and the reduced electric conductivity of the base layer material when coating the bulk layer material, particularly when using sputtering techniques.
Trying to reduce the dimensions of the metallic electric conductors up to now lead to dramatically degradation of the mechanical properties of the waveguides.